Transversely-excited film bulk acoustic resonators with electrodes having a second layer of variable width

ABSTRACT

There is disclosed acoustic resonators and filter devices. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces, the back surface attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The interleaved fingers include a first layer having a rectangular shape adjacent the diaphragm and a second layer over the first layer opposite the diaphragm, and wherein a width of the second layer varies along a length of each finger.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application63/041,052, filed Jun. 18, 2020, entitled XBAR WITH REDUCED LEVELS OFSPURIOUS MODE. This application is incorporated herein by reference.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to bandpass filters for use incommunications equipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is less than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies proposed for future communications networks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 3B is another alternative schematic cross-sectional view of theXBAR of FIG. 1.

FIG. 3C is an alternative schematic plan view of an XBAR

FIG. 4 is a graphic illustrating a primary acoustic mode in an XBAR.

FIG. 5 is a schematic circuit diagram of a band-pass filter usingacoustic resonators in a ladder circuit.

FIG. 6 is a cross-sectional view and two detailed cross-sectional viewsof a portion of an XBAR with two-layer interdigital transducer (IDT)fingers.

FIG. 7A is a plan view of a two-layer IDT finger with an upper layerextending along a length of the finger in a trapezoidal shape.

FIG. 7B is a plan view of a two-layer IDT finger with an upper layerextending along a length of the finger in an hourglass shape.

FIG. 7C is a plan view of a two-layer IDT finger with an upper layerextending along a length of the finger in a notched shape.

FIG. 7D is a plan view of a two-layer IDT finger with an upper layerextending along a length of the finger in another trapezoidal shape.

FIG. 7E is a plan view of a two-layer IDT finger with an upper layerextending along a length of the finger in a half hourglass shape.

FIG. 7F is a plan view of a two-layer IDT finger with an upper layerextending along a length of the finger in an elongated hexagonal shape.

FIG. 7G is a plan view of a two-layer IDT finger with an upper layerextending along a length of the finger in an irregular shape.

FIG. 8 is a plan chart of a process for fabricating an XBAR or a filterincluding XBARs.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the front and back surfaces 112, 114.However, XBARs may be fabricated on piezoelectric plates with othercrystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of the substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate around at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. As will be discussed in further detail, theprimary acoustic mode is a bulk shear mode where acoustic energypropagates along a direction substantially orthogonal to the surface ofthe piezoelectric plate 110, which is also normal, or transverse, to thedirection of the electric field created by the IDT fingers. Thus, theXBAR is considered a transversely-excited film bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the portion 115 of thepiezoelectric plate that spans, or is suspended over, the cavity 140. Asshown in FIG. 1, the cavity 140 has a rectangular shape with an extentgreater than the aperture AP and length L of the IDT 130. A cavity of anXBAR may have a different shape, such as a regular or irregular polygon.The cavity of an XBAR may more or fewer than four sides, which may bestraight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds ofparallel fingers in the IDT 110. Similarly, the thickness of the fingersin the cross-sectional views is greatly exaggerated.

The dimensions of an XBAR scale inversely with frequency. For example,the resonance frequency of an XBAR can be reduced by 20% by increasingall of the dimensions of an XBAR by 20%. Since the resonance frequencyof an XBAR is primarily determined by the thickness of the piezoelectricplate, it is convenient to express others dimensions relative to thepiezoelectric plate thickness.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. Thickness ts may be, for example, 100 nmto 1500 nm. When used in filters for LTE™ bands from 3.4 GHz to 6 GHz(e.g. bands 42, 43, 46), the thickness ts may be, for example, 200 nm to1000 nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 may be formed only between the IDT fingers (e.g. IDT finger238 b) or may be deposited as a blanket layer such that the dielectriclayer is formed both between and over the IDT fingers (e.g. IDT finger238 a). The front-side dielectric layer 214 may be a non-piezoelectricdielectric material, such as silicon dioxide or silicon nitride. tfd maybe, for example, 0 to 500 nm. tfd is typically less than the thicknessts of the piezoelectric plate. The front-side dielectric layer 214 maybe formed of multiple layers of two or more materials.

The IDT fingers 238 a and 238 b may be aluminum, an aluminum alloy,copper, a copper alloy, beryllium, gold, tungsten, molybdenum or someother conductive material. The IDT fingers are considered to be“substantially aluminum” if they are formed from aluminum or an alloycomprising at least 50% aluminum. The IDT fingers are considered to be“substantially copper” if they are formed from copper or an alloycomprising at least 50% copper. The IDT fingers are considered to be“substantially molybdenum” if they are formed from molybdenum or analloy comprising at least 50% molybdenum. Thin (relative to the totalthickness of the conductors) layers of other metals, such as chromium ortitanium, may be formed under and/or over and/or as layers within thefingers to improve adhesion between the fingers and the piezoelectricplate 110 and/or to passivate or encapsulate the fingers and/or toimprove power handling. The busbars (132, 134 in FIG. 1) of the IDT maybe made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The geometry of the IDT of an XBAR differs substantially fromthe IDTs used in surface acoustic wave (SAW) resonators. In a SAWresonator, the pitch of the IDT is one-half of the acoustic wavelengthat the resonance frequency. Additionally, the mark-to-pitch ratio of aSAW resonator IDT is typically close to 0.5 (i.e. the mark or fingerwidth is about one-fourth of the acoustic wavelength at resonance). Inan XBAR, the pitch p of the IDT is typically 2 to 20 times the width wof the fingers. In addition, the pitch p of the IDT is typically 2 to 20times the thickness is of the piezoelectric slab 212. The width of theIDT fingers in an XBAR is not constrained to be near one-fourth of theacoustic wavelength at resonance. For example, the width of XBAR IDTfingers may be 500 nm or greater, such that the IDT can be readilyfabricated using optical lithography. The thickness tm of the IDTfingers may be from 100 nm to about equal to the width w. The thicknessof the busbars (132, 134 in FIG. 1) of the IDT may be the same as, orgreater than, the thickness tm of the IDT fingers.

FIG. 3A and FIG. 3B show two alternative cross-sectional views along thesection plane A-A defined in FIG. 1. In XBAR 300 of FIG. 3A, apiezoelectric plate 310 is attached to a substrate 320. A portion of thepiezoelectric plate 310 forms a diaphragm 315 spanning a cavity 340 inthe substrate. The cavity 340 does not fully penetrate the substrate320. Fingers of an IDT are disposed on the diaphragm 315. The cavity 340may be formed, for example, by etching the substrate 320 beforeattaching the piezoelectric plate 310. Alternatively, the cavity 340 maybe formed by etching the substrate 320 with a selective etchant thatreaches the substrate (or sacrificial etch material in a patternedsubstrate) through one or more openings (not shown) provided in thepiezoelectric plate 310. In this case, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around a largeportion of a perimeter 345 of the cavity 340. For example, the diaphragm315 may contiguous with the rest of the piezoelectric plate 310 aroundat least 50% of the perimeter 345 of the cavity 340. An intermediatelayer (not shown), such as a dielectric bonding layer, may be presentbetween the piezo electric plate 340 and the substrate 320.

In in XBAR 300′ of FIG. 3B, the substrate 320 includes a base 322 and anintermediate layer 324 disposed between the piezoelectric plate 310 andthe base 322. For example, the base 322 may be silicon and theintermediate layer 324 may be silicon dioxide or silicon nitride or someother material. A portion of the piezoelectric plate 310 forms adiaphragm 315 spanning a cavity 340 in the intermediate layer 324.Fingers of an IDT are disposed on the diaphragm 315. The cavity 340 maybe formed, for example, by etching the intermediate layer 324 beforeattaching the piezoelectric plate 310. Alternatively, the cavity 340 maybe formed by etching the intermediate layer 324 with a selective etchantthat reaches the substrate through one or more openings provided in thepiezoelectric plate 310. In this case, the diaphragm 315 may becontiguous with the rest of the piezoelectric plate 310 around a largeportion of a perimeter 345 of the cavity 340 (see FIG. 3C). For example,the diaphragm 315 may be contiguous with the rest of the piezoelectricplate 310 around at least 50% of the perimeter 345 of the cavity 340 asshown in FIG. 3C. Although not shown in FIG. 3B, a cavity formed in theintermediate layer 324 may extend into the base 322.

FIG. 3C is a schematic plan view of another XBAR 350. The XBAR 350includes an IDT formed on a piezoelectric plate 310. A portion of thepiezoelectric plate 310 forms a diaphragm spanning a cavity in asubstrate. In this example, the perimeter 345 of the cavity has anirregular polygon shape such that none of the edges of the cavity areparallel, nor are they parallel to the conductors of the IDT. A cavitymay have a different shape with straight or curved edges.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430 which alternate in electrical polarity from finger to finger. An RFvoltage is applied to the interleaved fingers 430. This voltage createsa time-varying electric field between the fingers. The direction of theelectric field is predominantly lateral, or parallel to the surface ofthe piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the RF electric energy is highly concentratedinside the plate relative to the air. The lateral electric fieldintroduces shear deformation which couples strongly to a shear primaryacoustic mode (at a resonance frequency defined by the acoustic cavityformed by the volume between the two surfaces of the piezoelectricplate) in the piezoelectric plate 410. In this context, “sheardeformation” is defined as deformation in which parallel planes in amaterial remain predominantly parallel and maintain constant separationwhile translating (within their respective planes) relative to eachother. A “shear acoustic mode” is defined as an acoustic vibration modein a medium that results in shear deformation of the medium. The sheardeformations in the XBAR 400 are represented by the curves 460, with theadjacent small arrows providing a schematic indication of the directionand relative magnitude of atomic motion at the resonance frequency. Thedegree of atomic motion, as well as the thickness of the piezoelectricplate 410, have been greatly exaggerated for ease of visualization.While the atomic motions are predominantly lateral (i.e. horizontal asshown in FIG. 4), the direction of acoustic energy flow of the excitedprimary acoustic mode is substantially orthogonal to the surface of thepiezoelectric plate, as indicated by the arrow 465.

Considering FIG. 4, there is essentially no RF electric fieldimmediately under the IDT fingers 430, and thus acoustic modes are onlyminimally excited in the regions 470 under the fingers. There may beevanescent acoustic motions in these regions. Since acoustic vibrationsare not excited under the IDT fingers 430, the acoustic energy coupledto the IDT fingers 430 is low (for example compared to the fingers of anIDT in a SAW resonator) for the primary acoustic mode, which minimizesviscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. Such devices are usually based on AlN thinfilms with the C axis of the AlN perpendicular to the surfaces of thefilm. The acoustic mode is compressive with atomic motions and thedirection of acoustic energy flow in the thickness direction. Inaddition, the piezoelectric coupling for shear wave XBAR resonances canbe high (>20%) compared to other acoustic resonators. High piezoelectriccoupling enables the design and implementation of microwave andmillimeter-wave filters with appreciable bandwidth.

FIG. 5 is a schematic circuit diagram of a band-pass filter 500 usingfive XBARs X1-X5. The filter 500 may be, for example, a band n79band-pass filter for use in a communication device. The filter 500 has aconventional ladder filter architecture including three seriesresonators X1, X3, X5 and two shunt resonators X2, X4. The three seriesresonators X1, X3, X5 are connected in series between a first port and asecond port. In FIG. 5, the first and second ports are labeled “In” and“Out”, respectively. However, the filter 500 is bidirectional and eitherport may serve as the input or output of the filter. The two shuntresonators X2, X4 are connected from nodes between the series resonatorsto ground. All the shunt resonators and series resonators are XBARs.

The three series resonators X1, X3, X5 and the two shunt resonators X2,X4 of the filter 500 may be formed on a single plate 530 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 5, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 535). In this example, an IDT of each resonator isdisposed over a respective cavity. In other filters, the IDTs of two ormore resonators may be disposed over a common cavity. Resonators mayalso be cascaded into multiple IDTs which may be formed on multiplecavities.

Each of the resonators X1 to X5 has a resonance frequency and ananti-resonance frequency. In simplified terms, each resonator iseffectively a short circuit at its resonance frequency and effectivelyan open circuit at its anti-resonance frequency. Each resonator X1 to X5creates a “transmission zero”, where the transmission between the in andout ports of the filter is very low. Note that the transmission at a“transmission zero” is not actually zero due to energy leakage throughparasitic components and other effects. The three series resonators X1,X3, X5 create transmission zeros at their respective anti-resonancefrequencies (where each resonator is effectively an open circuit). Thetwo shunt resonators X2, X4 create transmission zeros at theirrespective resonance frequencies (where each resonator is effectively ashort circuit). In a typical band-pass filter using acoustic resonators,the anti-resonance frequencies of the series resonators createtransmission zeros above the passband, and the resonance frequencies ofthe shunt resonators create transmission zeros below the passband.

A band-pass filter for use in a communications device, such as acellular telephone, must meet a variety of requirements. First, aband-pass filter, by definition, must pass, or transmit with acceptableloss, a defined pass-band. Typically, a band-pass filter for use in acommunications device must also stop, or substantially attenuate, one ormore stop band(s). For example, a band n79 band-pass filter is typicallyrequired to pass the n79 frequency band from 4400 MHz to 5000 MHz and tostop the 5 GHz WiFi™ band and/or the n77 band from 3300 MHz to 4200 MHz.To meet these requirements, a filter using a ladder circuit wouldrequire series resonators with anti-resonance frequencies about or above5100 MHz, and shunt resonators with resonance frequencies about or below4300 MHz.

Another typical requirement on a band-pass filter for use in acommunications device is the input and output impedances of the filterhave to match, at least over the pass-band of the filter, the impedancesof other elements of the communications device to which the filter isconnected (e.g. a transmitter, receiver, and/or antenna) for maximumpower transfer. Commonly, the input and output impedances of a band-passfilter are required to match a 50-ohm impedance within a tolerance thatmay be expressed, for example, as a maximum return loss or a maximumvoltage standing wave ratio. When necessary, an impedance matchingnetwork comprising one or more reactive components can be used at theinput and/or output of a band-pass filter. Such impedance matchingnetworks add to the complexity, cost, and insertion loss of the filterand are thus undesirable. To match, without additional impedancematching components, a 50-Ohm impedance at a frequency of 5 GHz, thecapacitances of at least the shunt resonators in the band-pass filterneed to be in a range of about 0.5 picofarads (pF) to about 1.5picofarads.

The metal fingers of the IDTs provide the primary mechanism for removingheat from an XBAR resonator. Increasing the aperture of a resonatorincreases the length and the electrical and thermal resistance of eachIDT finger. Further, for a given IDT capacitance, increasing theaperture reduces the number of fingers required in the IDT, which, inturn, proportionally increases the RF current flowing in each finger.All of these effects argue for using the smallest possible aperture inresonators for high-power filters.

Conversely, several factors argue for using a large aperture. First, thetotal area of an XBAR resonator includes the area of the IDT and thearea of the bus bars. The area of the bus bars is generally proportionalto the length of the IDT. For very small apertures, the area of the IDTbus bars may be larger than the area occupied by the interleaved IDTfingers. Further, some electrical and acoustic energy may be lost at theends of the IDT fingers. These loss effects become more significant asIDT aperture is reduced and the total number of fingers is increased.These losses may be evident as a reduction in resonator Q-factor,particularly at the anti-resonance frequency, as IDT aperture isreduced.

As a compromise between conflicting objectives, resonator apertures willtypically fall in the range from 20 μm and 60 μm for 5 GHz resonancefrequency. Resonator aperture may scale inversely with frequency.

Communications devices operating in time-domain duplex (TDD) bandstransmit and receive in the same frequency band. Both the transmit andreceive signal paths pass through a common bandpass filter connectedbetween an antenna and a transceiver. Communications devices operatingin frequency-domain duplex (FDD) bands transmit and receive in differentfrequency bands. The transmit and receive signal paths pass throughseparate transmit and receive bandpass filters connected between anantenna and the transceiver. Filters for use in TDD bands or filters foruse as transmit filters in FDD bands can be subjected to radio frequencyinput power levels of 30 dBm or greater and must avoid damage underpower.

The required insertion loss of acoustic wave bandpass filters is usuallynot more than a few dB. Some portion of this lost power is return lossreflected back to the power source; the rest of the lost power isdissipated in the filter. Typical band-pass filters for LTE bands havesurface areas of 1.0 to 2.0 square millimeters. Although the total powerdissipation in a filter may be small, the power density can be highgiven the small surface area. Further, the primary loss mechanisms in anacoustic filter are resistive losses in the conductor patterns andacoustic losses in the IDT fingers and piezoelectric material. Thus, thepower dissipation in an acoustic filter is concentrated in the acousticresonators. To prevent excessive temperature increase in the acousticresonators, the heat due to the power dissipation must be conducted awayfrom the resonators through the filter package to the environmentexternal to the filter.

In traditional acoustic filters, such as surface acoustic wave (SAW)filters and bulk acoustic wave (BAW) filters, the heat generated bypower dissipation in the acoustic resonators is efficiently conductedthrough the filter substrate and the metal electrode patterns to thepackage. In an XBAR device, the resonators are disposed on thinpiezoelectric diaphragms that are inefficient heat conductors. The largemajority of the heat generated in an XBAR device must be removed fromthe resonator via the IDT fingers and associated conductor patterns.

The electric resistance of the IDT fingers can be reduced, and thethermal conductivity of the IDT fingers can be increased, by increasingthe cross-sectional area of the fingers to the extent possible. Asdescribed in conjunction with FIG. 4, unlike SAW or BAW, for XBAR thereis little coupling of the primary acoustic mode to the IDT fingers.Changing the width and/or thickness of the IDT fingers has minimaleffect on the primary acoustic mode in an XBAR device. This is a veryuncommon situation for an acoustic wave resonator. However, the IDTfinger geometry does have a substantial effect on coupling to spuriousacoustic modes, such as higher order shear modes and plate modes thattravel laterally in the piezoelectric diaphragm.

FIG. 6 is a cross-sectional view of a portion of an XBAR with two-layerIDT fingers. FIG. 6 shows a cross section though a portion of apiezoelectric diaphragm 610 and two IDT fingers 620. Each IDT finger 620has two metal layers 630, 640. The lower (as shown in FIG. 6) layer 630may be a metal with low acoustic impedance, such as aluminum, copper,molybdenum, silver, or titanium, e.g., a metal with low transverseacoustic impedance. Transverse acoustic impedance is the product ofdensity and shear wave velocity. The lower layer 630 may be adjacent thediaphragm 610 or separated from the diaphragm 610 by a thin intermediatelayer (not shown) used to improve adhesion between the diaphragm 610 andthe lower layer 630. The upper layer 640 may be a metal with highthermal and electrical conductivity, such as aluminum, copper, or gold.The use of a metal with low acoustic impedance for the lower layer 630closest to the diaphragm 610, where the acoustic stresses are greatest,reduces acoustic losses in the XBAR. Having two metal layers 630, 640allows the designer to have additional design options to further improveperformance of the XBAR.

Further, the two metal layers need not have the same thickness orcross-sectional shape, as shown in Detail A and Detail B of FIG. 6. InDetail A, the second metal layer 640A of each IDT finger has the form ofa narrow rib on top of a thinner, wider first metal layer 630A. InDetail B, each IDT finger has a “T” cross section form by a narrow firstmetal layer 630B and a wider second metal layer 640B. The cross-sectionshapes of the first and second metal layers are not limited torectangular as shown in FIG. 6. Other cross-sectional shapes includingtrapezoidal and triangular may be used and may be beneficial to minimizeor control spurious acoustic modes.

FIGS. 7A-G are plan views of exemplary two-layer XBAR IDT fingers withvarious shapes, which may reduce or modify spurious modes as comparedwith the XBARs with rectangular shapes. In each example, the lower layeradjacent the diaphragm is a rectangle (shown in dashed lines) withconstant width. The upper layer has a significantly varying width alongthe length of the finger, preventing generation of resonances inside thefinger at a single frequency and synchronous reflection of thepropagating modes. The width w and finger length are identified in FIG.7A.

In single layer fingers, some spurious modes can be reduced by carefuladjustment of the electrode height and width, but only in limitedfrequency range that is not sufficient for a wide-band filter. There isno efficient way for mitigating the propagation of Lamb modes, which areusually high order harmonics (7th-13th, etc.) of S0 and A0.

In the two-layer fingers of FIGS. 7A-G, a constant width of the lowerlayer of the finger provides a uniform electric field between fingersand uniform excitation of the shear primary acoustic mode across theaperture. The variable width of the top layer of the finger along thelength of the finger disrupts internal resonances inside the finger suchthat metal resonances will be mitigated. This will also reducesynchronous reflection and increase scattering of propagating modes,thus reducing their amplitude. An acoustic wave propagating in anelectrode structure with constant periodicity tends to resonate at afrequency determined by the velocity of the wave and the periodicity ofthe structure. In XBAR devices, such resonances are undesireddisturbances. By varying the width of the upper electrode layer, theresonance occurs at a different frequency in different locations alongthe finger, thus reducing a magnitude of the response.

The variation in width of the upper layer should be around half of theLN membrane thickness. The lower layer and the upper layer can be formedof the same or different materials. For example, the thickness of thelower layer could be in a range from 25% to 75% of the thickness of thediaphragm, and the thickness of the upper layer could be in a range from25% to 75% of the thickness of the diaphragm. The upper layer and thelower layer can be formed of the same or different materials. The upperlayer can cover the busbars to improve the heat transfer. In otherexamples, the fingers can have additional layers, such as three or fourlayers. A passivation layer can be formed on top of the fingers.

FIG. 7A is a plan view of a two-layer IDT finger 700A with an upperlayer 720B extending along a length of the finger in a trapezoidal shapeon a lower layer 710A with a constant width. The upper layer 720A tapersfrom a width about equal to the lower layer 710A at one end to a widthabout half of the lower layer 710A at the other end.

FIG. 7B is a plan view of a two-layer IDT finger 700B with an upperlayer 720B extending along a length of the finger in an hourglass shapeon a lower layer 710B of constant width. The upper layer 720B tapersfrom a width about equal to the lower layer 710B at one end, to a widthabout half of the lower layer 710B at a midpoint, to a width about equalto the lower layer 710B at the other end.

FIG. 7C is a plan view of a two-layer IDT finger 700C with an upperlayer 720C extending along a length of the finger in a notched shape ona lower layer 710C of constant width. A width of the upper layer 720C isabout equal to a width of the lower layer 710C at portions and equal toabout half the width of the lower layer 710C at alternating portionswith notches on opposing sides.

FIG. 7D is a plan view of a two-layer IDT finger 700D with an upperlayer 720 extending along a length of the finger in another trapezoidalshape on a lower layer 710D of constant width. A width of the upperlayer 720D tapers on one side so that a width of the upper layer 720D isabout the width of the lower layer 710D at one end and is less than awidth of the lower layer 710D at the other end.

FIG. 7E is a plan view of a two-layer IDT finger 700E with an upperlayer 720E extending along a length of the finger in a half hourglassshape on a lower layer 710E of constant width. A width of the upperlayer 720E tapers on one side so that a width of the upper layer 720E isabout the width of the lower layer 710E at both ends and is less than awidth of the lower layer 710D at a midpoint.

FIG. 7F is a plan view of a two-layer IDT finger 700F with an upperlayer 720F extending along a length of the finger in an elongatedhexagonal shape on a lower layer 710F of constant width. A width of theupper layer 720F tapers on both sides so that a width of the upper layer720E is less than the width of the lower layer 710E at both ends and isabout equal to a width of the lower layer 710D at a midpoint.

FIG. 7G is a plan view of a two-layer IDT finger 700G with an upperlayer 720G extending along a length of the finger in an irregular shapeon a lower layer 710G of constant width. A width of the upper layervaries along the length of the finger.

FIG. 7A through FIG. 7G are examples of a nearly unlimited number ofpossible upper electrode layer shapes. The upper layer can have anyshape that varies along the length of each IDT finger. For example, theedges of the upper layer can be straight (as shown in FIG. 7A to FIG.7G) or curved. The edges of the shapes of can also be a mix of curvedand straight edges. The angles between straight edges can be greater orless than those shown in the figures. Two or more different upper layershapes can be intermixed on the fingers of a single IDT and differentupper layer shapes or combinations of shapes can be used on differentacoustic resonators in a filter. For example, the shape of the upperlayer can vary between different fingers of a single IDT or differentIDTs in a device.

Description of Methods

FIG. 8 is a simplified flow chart showing a process 800 for making anXBAR or a filter incorporating XBARs. The process 800 starts at 805 witha substrate and a plate of piezoelectric material and ends at 895 with acompleted XBAR or filter. The flow chart of FIG. 8 includes only majorprocess steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 8.

The flow chart of FIG. 8 captures three variations of the process 800for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 810A, 810B, or 810C.Only one of these steps is performed in each of the three variations ofthe process 800.

The piezoelectric plate may be, for example, rotated Z-cut lithiumniobate. The Euler angles of the piezoelectric plate are 0, β, 90°,where β is in the range from −15° to +5°. Preferably, β may be in therange from −11° to −5° to maximize electromechanical coupling. β may bein the range from −10° to −7.5° to maximize Q-factor at the resonancefrequency. The substrate may preferably be silicon. The substrate may besome other material that allows formation of deep cavities by etching orother processing, or has other tradeoffs in mechanical performance orcost.

In one variation of the process 800, one or more cavities are formed inthe substrate at 810A, before the piezoelectric plate is bonded to thesubstrate at 820. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 810A will not penetrate through the substrate.

At 820, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. When a dielectric material is sandwichedbetween the piezoelectric plate and the substrate during bonding, thedielectric material may subsequently be left in place or removed. One orboth mating surfaces may be activated using, for example, a plasmaprocess. The mating surfaces may then be pressed together withconsiderable force to establish molecular bonds between thepiezoelectric plate and the substrate or intermediate material layers.

A lower conductor pattern, including the lower layers of the IDTs ofeach XBAR, is formed at 830 by depositing and patterning one or morelower conductor layer on the front side of the piezoelectric plate. Thelower conductor layer may be, for example, aluminum, an aluminum alloy,copper, a copper alloy, or some other conductive metal. Optionally, oneor more layers of other materials may be disposed below (i.e. betweenthe conductor layer and the piezoelectric plate) and/or on top of theconductor layer. For example, a thin film of titanium, chrome, or othermetal may be used to improve the adhesion between the lower conductorlayer and the piezoelectric plate.

The lower conductor pattern may be formed at 830 by depositing the lowerconductor layer and, optionally, one or more other metal layers insequence over the surface of the piezoelectric plate. The excess metalmay then be removed by etching through patterned photoresist. The lowerconductor layer can be etched, for example, by plasma etching, reactiveion etching, wet chemical etching, and other etching techniques.

Alternatively, the lower conductor pattern may be formed at 830 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The lowerconductor layer and, optionally, one or more other layers may bedeposited in sequence over the surface of the piezoelectric plate. Thephotoresist may then be removed, which removes the excess material,leaving the lower conductor pattern.

An upper conductor pattern, including the upper layers of the IDTs ofeach XBAR, is formed at 835 by depositing and patterning one or moreupper conductor layer on the front side of the piezoelectric plate. Theupper conductor pattern may be formed using the same techniquesdescribed for the lower conductor pattern with a different mask.

When the device has additional conductor layers, the layers may bedeposited and patterned separately. In particular, different patterningprocesses (i.e. etching or lift-off) may be used on different layers anddifferent masks are required where two or more layers have differentwidths or shapes.

At 840, a front-side dielectric layer may be formed by depositing one ormore layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 800, one or more cavities areformed in the back side of the substrate at 810B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 800, a back-side dielectric layermay be formed at 850. In the case where the cavities are formed at 810Bas holes through the substrate, the back-side dielectric layer may bedeposited through the cavities using a conventional deposition techniquesuch as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 800, one or more cavities in theform of recesses in the substrate may be formed at 810C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device.

In all variations of the process 800, the filter device is completed at860. Actions that may occur at 860 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at 860is to tune the resonant frequencies of the resonators within the deviceby adding or removing metal or dielectric material from the front sideof the device. After the filter device is completed, the process ends at895.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface; a single-crystal piezoelectric plate having front andback surfaces, the back surface attached to the surface of the substrateexcept for a portion of the single-crystal piezoelectric plate forming adiaphragm that spans a cavity in the substrate; and an interdigitaltransducer (IDT) formed on the front surface of the single-crystalpiezoelectric plate, interleaved fingers of the IDT disposed on thediaphragm, wherein the interleaved fingers comprise a first layeradjacent the diaphragm and a second layer over the first layer oppositethe diaphragm, and wherein a width of the second layer varies along alength of each finger.
 2. The device of claim 1, wherein thesingle-crystal piezoelectric plate and IDT are configured such that aradio frequency signal applied to the IDT excites a primary shearacoustic mode in the diaphragm.
 3. The device of claim 1, wherein ashape of the second layer of a first interleaved finger is differentfrom a shape of the second layer of a second interleaved finger.
 4. Thedevice of claim 1, wherein the width of the second layer decreaseslinearly along at least a portion of a length of the second layer. 5.The device of claim 1, wherein an edge of the second layer is curved. 6.The device of claim 1, wherein the width of the second layer increasesalong a first portion of the second layer and decreases along a secondportion of the second layer.
 7. The device of claim 1, wherein athickness of the first layer is less than a thickness of the secondlayer.
 8. The device of claim 1, wherein a width of the first layer isconstant along a length of the interleaved fingers.
 9. The device ofclaim 1, wherein the diaphragm is contiguous with the single-crystalpiezoelectric plate around at least 50% of a perimeter of the cavity.10. The device of claim 1 further comprising an adhesion layer betweenthe first layer and the diaphragm.
 11. A filter device comprising: asubstrate having a surface; a single-crystal piezoelectric plate havingfront and back surfaces, the back surface attached to the surface of thesubstrate, portions of the single-crystal piezoelectric plate formingone or more diaphragms spanning respective cavities in the substrate;and a conductor pattern formed on the front surface, the conductorpattern including a plurality of interdigital transducers (IDTs) of arespective plurality of acoustic resonators, interleaved fingers of eachof the plurality of IDTs disposed on one of the one or more diaphragms,wherein each of the interleaved fingers comprises a first layer adjacentthe one or more diaphragms and a second layer over the first layeropposite the one or more diaphragms, and wherein a width of the secondlayer varies along a length of each finger.
 12. The device of claim 11,wherein the single-crystal piezoelectric plate and all of the IDTsconfigured such that respective radio frequency signals applied to eachIDT excite respective shear primary acoustic modes in the respectivediaphragm.
 13. The device of claim 11, wherein a shape of the secondlayer of a first interleaved finger of the interleaved fingers of one ofthe plurality of IDTs is different from a shape of the second layer of asecond interleaved finger of the interleaved fingers of the one of theplurality of IDTs.
 14. The device of claim 11, wherein the width of thesecond layer of each of the interleaved fingers of at least one of theplurality of IDTs decreases linearly along at least a portion of alength of the second layer.
 15. The device of claim 11, wherein an edgeof the second layer of each of the interleaved fingers of at least oneof the plurality of IDTs is curved.
 16. The device of claim 11, whereinthe width of the second layer of each of the interleaved fingers of atleast one of the plurality of IDTs increases along a first portion ofthe second layer and decreases along a second portion of the secondlayer.
 17. The device of claim 11, wherein a thickness of the firstlayer of each of the interleaved fingers of each of the plurality ofIDTs is less than a thickness of the second layer.
 18. The device ofclaim 11, wherein a width of the first layer of each of the interleavedfingers of each of the plurality of IDTs is constant along a length ofthe interleaved fingers.
 19. The device of claim 11, wherein each of theone or more diaphragms is contiguous with the single-crystalpiezoelectric plate around at least 50% of a perimeter of the respectivecavity.
 20. The device of claim 11 comprising an adhesion layer betweenthe first layer of each of the interleaved fingers of each of theplurality of IDTs and the one or more diaphragms.
 21. The device ofclaim 11, wherein the plurality of acoustic resonators comprises atleast one first acoustic resonator and at least one second acousticresonator, wherein a front side dielectric layer is deposited over theat least one first acoustic resonator and not deposited over the atleast one second acoustic resonator.
 22. A method of fabricating anacoustic resonator device, comprising: bonding a piezoelectric plate toa substrate; forming a cavity in the substrate, before or after bondingthe piezoelectric plate to the substrate, such that a portion of thepiezoelectric plate forms a diaphragm spanning the cavity; and formingan interdigital transducer (IDT) on the front surface of thepiezoelectric plate such that interleaved fingers of the IDT aredisposed on the diaphragm, wherein the interleaved fingers comprise afirst layer having a rectangular shape adjacent the diaphragm and asecond layer over the first layer opposite the diaphragm, and wherein awidth of the second layer varies along a length of each finger.
 23. Themethod of claim 22, wherein the piezoelectric plate and IDT areconfigured such that a radio frequency signal applied to the IDT excitesa primary shear acoustic mode in the diaphragm.
 24. The method of claim23, wherein a shape of the second layer of a first interleaved finger ofthe IDT is different from a shape of the second layer of a secondinterleaved finger of the IDT.
 25. The method of claim 22, wherein thewidth of the second layer decreases linearly along at least a portion ofa length of the second layer.
 26. The method of claim 22, wherein anedge of the second layer is curved.
 27. The method of claim 22, whereinthe width of the second layer increases along a first portion of thesecond layer and decreases along a second portion of the second layer.28. The method of claim 22, wherein a thickness of the first layer isless than a thickness of the second layer.
 29. The method of claim 22,wherein a width of the first layer is constant along a length of theinterleaved fingers.
 30. The method of claim 22, wherein the diaphragmis contiguous with the piezoelectric plate around at least 50% of aperimeter of the cavity.
 31. The method of claim 22 further comprisingforming an adhesion layer between the first layer and the diaphragm.